Frequency multiplier



Patented Nov. 11, 1941 FREQUENCY MUL'ri-P' mn Henry M. Bach, Woodmere,N. YQ, assignor to Radio Patents Corporation, a corporation of New York Application November 5, 1940, Serial No. 364,353 13 Claims. (01. 25036) The present invention relates to frequency multipliers, more particularly to an improved frequency doubler utilizing an electron discharge tube as a frequency changing element.

The novel frequency doubling circuit provided by the invention has special use, though by no means limited thereto, in radio transmitters, in particular frequency modulated transmitters, in enhancing both the efliciency of the transmitter and eliminating distortions and other defects inherent in the arrangements heretofore used. I

With the advent of wide band frequency modulation the necessity for :frequency multiplier circuits that will operate efliciently and at the same time require a minimum of driving .power and component parts has arisen. Frequency multipliers commonly used at the present time are of the distortion type. In one embodiment of this type of frequency multiplier an electron r discharge tube is operated far in the class C region so that the angle of plate currentflow is favorable for generating ajdesired harmonic of the fundamental frequency. For frequency doubling the optimum angle of plate current flow is 90.

For an angle ofl35 the output will be increasedslightly but this is more than compensated for by a disproportionate increase in input power. A second type of harmonic generator known as grid distortion multiplier makes 2 use of the non-linear relationship between grid current and voltage to distort the voltage .applied to the grid and hence to apply harmonic components to the plate circuit. This method also makes it necessary to operate the tube in the I Val frequency modulated broadcasting stations at the present time, an initial phase modulated carrier Wave with a 30 maximum deviation at a frequency of about 200 kc. is frequency multiplied,

heterodyned down. and then multiplied again in i order to secure a wide band frequency modulation with a frequency deviation of :75 kc. in the 42-50 megacycle band assigned to frequency modulation. This method requires a great number of frequency doubler stages and design praci tice dictates that these stages operate at very low energy levels. Accordingly, small receiving tubes are used throughout the multiplier stages. In these arrangements difficulties have been en countered among others to eliminate undesirable I harmonic and fundamental phas or frequency modulation from passing through the system and appearing in the output as spurious modulation components with attendant signal distortion and other defects. The ease with which these distortion components may be generated will be appreciated when it is realized that the doubler stagesLhave to be of the bandpass type so as to allow the full frequency band caused by the modulation to be transmitted without side band attenuation iri order to preventamplitude modulation due to non-linear outputfrequency relationship at the extremities of the frequency modulation swing. Similar difficulties and defects are encountered in other types of frequency modulated transmitters utilizing aninitial low frequency being stepped up to thefinal operating frequency radiated by the transmitter in order to obtain wide bandfrequency modulation in accordance with present standards As pointed out, the known doublers require that the grids of the tubes be driven positive and since small receiving tubes as used in the doubler stages are not designed to operate with grid current, the systems are deficient in many respects. At any rate, grid current flow is not conducive to long tube lif for smallreceiving tubes. Accordingly an object of the invention is-the provision of a new electron tube frequency doubler circuit which does not require any. driving power, that is which is operated substantially within the class A operating region of the A further doubler circuit which will furnish a high output free from spurious frequencycomponents and which is both simple in design and capable of operating with a minimum of input power.

Other objects and advantages of the invention vwill become more apparent from the following detailed description taken with reference to the accompanying drawing forming part of this specification andwherein: I

Figure 1 is a basic electron tube circuit as used by the invention shown for explaining the principle and operation ofthe invention,

Figures 2 to 7 are circuit diagrams showing various modifications of practical frequency doubler circuits embodying the principles of the invention. q

Like reference numerals identify like parts throughout the different views of the drawing. Referring more particularly to Figure 1, there is shown a basic circuit explanatory of the prinple and function of theinvention comprising an object is to provide a frequency electron discharge tube It) having at least the following electrodes arranged in the order named: a cathode II, a first control grid l2, a second control grid l3, an acceleration or screen grid [4 enclosing the grid I3 and an anode or plate l5. The control grids l2 and I3 are negatively biased with respect to the cathode by the provision of a conventional biasing network It in the common cathode return lead for the grid and plate circuits, said network comprising a resistor by-passed by condenser, whereby the tube is operated in the class A operating region. The screen grid M and plate I5 are maintained at positive direct current potentials with respect to the cathode supplied by suitable supply sources indicated by the plus signs and the screen grid is grounded for alternating potentials by means of a by-pass condenser in a manner well understood. Item ll represents a load impedance inserted in the plate circuit the function of which will be described later.

In a systemaccording to Figure 1, the total electron space, current emitted by the cathode may be expressed by the following formula:

The platecurrent Ip will be proportional ,to the product of the grid voltages and may be expressed by the following theoretical equation:

1;= a+[ 1+ i61+ 1 1 1 1 i i i [E +A e;+B e +Czez?-t- 2f 2 wherein E1 and E2 are the steady biasing voltages and 1 and at are the alternating voltages applied to the control grids l2 and I3 and the cathode, respectively, and A1, B1, C1 N1 and A2, B2, C2 N2 are proportionality constants. The term In represents the steady or quiescent plate current which is not affected by the input potential on the control grids and is substantially a function of the geometry of the tube. The first series of terms in the equation is-due to the controlling action of the first control grid and the second series of terms is due to the controlling action of the second control grid.

By multiplying out Equation 2 there is obtained the following:

frequency but differ from each other by a constant phase angle that is wherein Isg' represents the screen grid current and then the term I) of Equation 3 will be as follows:

K (sin wt +sin (wt+) -2K sin wt cos g-t-K cos wt sin (4) and similarly term 0 of Equation 3 will be as follows K sin wt sin (wt-hp) =K sin wt (sin wt cos +cos wt sin 4;)

=K sin wt cos +K sin wt cos wt sin Inserting (4) and (5) in Equation 3 will result in a total plate current as follows:

=K sin wt cos dd sin 2wt sin I,,=I,,|2K sin wt cos +K cos wt sin +K sin wt cos od sin 2 wt sin (6) Assuming further that the phase angle between the grid voltages is the equation of the plate current will reduce to the following:

I,,=I,,+K sin wt+K cos wt+ sin 2 wt (7) hence for =90 Equation 7 shows that the plate current will contain the steady or quiescent component Io, two components containing the signals on the control grids, since for =90, cos wt:

sin (wt+), and a third term at twice the frequency of the input signal.

The plate current Ip will flow in the external circuit through the load impedance I1 and if the value Z of the latter is designed to ofier high impedance to currents of twice the input fre quency, the output voltage developed across the plate load impedance will be at twice the input frequency. This will be so since the Q value of the impedance I! will be such that the steady current In and the next two terms in Equation 7 will flow through the impedance unimpeded and hence will develop no output voltage. The last term, however, will develop a high output voltage (20 across the impedance 11 which output voltage is expressed as follows:

that is, the output will be at twice the input frequency.

Referring to Figure 2, there is shown a practical circuit diagram for a frequency doubler embodying the principles of the invention. The tube and its associated biasing and operating sources are substantially the same as in Figure 1. There is shown at IS a source of alternating cur rent which may be the master oscillator or the output of a preceding doubler stage in a radio transmitter. In order to obtain quadrature control voltages e1 and 62 to be impressed upon the control grids I2 and l 3, there is provided a phase shifting network consisting in the example shown of a pair of branch circuits shunted across the source l8 and each comprising avresistiveimpedance, 2:0 and 22; in series with a; reactive impedance such as condensers: 2|: and 23-, respectively. The impedance value of: the resistors and 22 are equal to each other and to the impedance ofiered by the condensers 2| and. 23 to the: alternating current supplied. by

the source i=8. In this: case: the. currents through both branch circuits, will be leading the impressed voltage by' 45 and since the control voltages for the: grids l2 and I3 are derived from the resistive and reactive dropsr developed across resistor 22 and condenser,respectively, they will be of equal magnitude and in quadrature phasev relation to each other; Alternatively, one. of the control potentials: may be derived directly from the source l8"whi'le the other control voltage is impressed from the source I18 to the respective control. grids: through anysuitable quadrature phase shift device known inthe art. such as a Single resistance-capacity series network.

A voltage of twice the input frequency is developed in the plate circuit from the third current term according to Equation '7 by the provision of a parallel tuned circuit 24 comprising an inductance and a condenser in parallel and being resonant to twice the input frequency;

Thus the circuit 21 will offer a high impedance tothis frequency resulting in a high potential drop being developedacross the circuit which maybe utilized in any suitable manner. In the a parallel tuned circuit resonant. to the frequency of theindiuced' current variations the voltage established between the grid I3 and the cathode willbe. in exact quadrature with the space current fluctuations thatisin turn with the voltage betweeniecntrollgrld I2 and cathode? due to the capacitative nature of the coupling between the space charge and the grid [3. In this manner the cohtroligrids i2 and I3 will: be excited by example illustrated, a secondary coupling coil 25' is provided for this purpose for transmitting output energy of twice the frequency either to a succeeding doubler stage or any otherutili'zation circuit; I r

Referring to Figure 3 there. isshown a doubler circuit similar to the preceding modification wherein, however, the 90 phaseangle between the control voltages is effected within the tube itself thereby preventing any interaction between the control actions and providing greater operating: stability and freedom from undesired components in the output circuit. To this end the voltage supplied from the source I8 is directly impressed upon one of the control grids in the example shown upon the first grid l2 and the cathode, while the second control grid is connected tothe cathode or ground through a nonreactive impedance providing a direct current path such as parallel tuned circuit comprising an inductance 26" and ondenser-r1 and being resonant to thefrequencyofthe source l8; A1- ternatively, this paralleltuned' circuit may be replaced by a high ohmic resistor. If a tuned circuit is employed, the same is carefully shielded from the remaining parts of the system such as by the aid of a screen 28 whereby the circuit is substantially free of any exterior coupling and is excited from the source F8 substantially by electron coupling by way of the electron stream to the presence of a so-called virtual cathode or concentratedspace charge setup in the vicinity of grid l3 as a result ofthe decelerating action of the latter on electrons passing through the openings of the accelerating grid !4. This space charge variesin intensity in accordance with the fluctuations of the discharge current that is in accordance with the frequency of the source l8 exciting the control grid: l2. Due to the presence of the variable space charge. a current will be induced in thegrid I3 flowing. through the external circuit connected thereto/and if" the latter offers resistiveimpedance onlyas in the case of passing through the tube. This coupling is due v voltages of'like frequency and quadrature phase relation resulting in energy of twice the input frequency being developedin the output circuit 24 in substantially'the same manner as" described hereinbefore.

Figure 4 shows a modification of Figure '3 wherein therfunction of the grids 12" and [3 are interchanged, that is the source 18 is connected to the grid 13 whereby itwill be found'thata space: charge couplingwill exist in the opposite direction,-that is from the grid l3 towards the grid. [21 through a resistive impedance in the example illustrated ahigh ohmic resistance 29 which may be replaced by a tuned circuit asinthe caseof Figure 3. Accordingly again, energy will be developed. at twice the frequencyin the output circuit 24 which may be fed to a succeedingdoubler stage or directly to a utilization circuit.

Fi urefi shows another modification for obtaining quadrature control voltages by the use of a double: tuned transformer or band pass filter comprising :11 primary circuit 30" and secondary circuit 3 I inductively coupled with each other and each being resonant to the frequency of the source l8; As. iswell known, the voltages developed acrossthe primary and secondary circuitsioi" a resonant transformer of this type are in quadrature relation and byconriecting each of the high potentialsidesof the circuits to one of thewcontrol. gridsythe latter will be excited in quadrature from the input source and output energy of twice thefrequency developedin the plate: circuit inthe manner described.

From: the foregoing itwill be further understood that the product term 0 in Equation 7f may be produced by means of devices or circuits other thana doublegrid electron tube employed in the preceding exemplifications; Devices or circuits of this type are known in the art as mixers or modbeing impressed upon' the remaining pair of apices of the bridgenetwork.

The-quadrature voltages maybe produced from the source lfl in an-y suitable manner such as by theiaidz of a; double tuned transformer 30, 3| provided: in the example: illustrated. The output circuit 24 resonant to twice the input frequency is connected tosuitable tap pointspreferably the electricall center points of the input circuits for the bridge network. In the example shown the output circuit is connected to the center tap points of the prim'ary and secondary windings of the tuned transformer whereby there will be produced-ma manner" well known an output current proportionate to theproductof the input The latter: is connected to ground voltages and hence includingJa component of twice the input frequency as is understood from the above.

Referring to Figure 7 there is shown a further embodiment of the invention utilizing a balanced modulator for obtaining a product function output current. The quadrature voltages derived from the source I8 are at first combined to form the sum and difference voltages rectified separately by means of suitable rectifying devices such as diode or triode vacuum tubes as shown. By differentially combining the rectified currents there is obtained a resultant being a product function of the original quadrature voltages and accordingly containing a component at twice the input frequency. In Figure 7 the voltage from source I8 is impressed upon the grids of a pair of triode rectifiers in' anti-phase relation by Way of a coupling transformer having a primary 3! and secondary 38. Furthermore a quadrature voltage derived from the source l8 by means of a phase shift network comprising resistor 42 and condenser 43 is impressed co-phasally upon the grids of the tubes 40 and M by way of the mid point of the secondary winding 38 whereby one of the tubes will be excited by the sum and the other tube will be excited by'the difference of the quadrature voltages. The rectified output currents of the tubes are combineddifferentially by connecting the anode together "and to the common output circuit, the latter including the tuned circuit 24 resonant at twice the input circuit to effect a frequency doubling in a manner understood from the foregoing.

It will be evident from the'foregoing that the invention is not limited to the specific circuits and arrangements of parts shown and disclosed herein for illustration but that the underlying concept and principle of the invention are susceptible of numerous variations and modifications coming within the broader scope and spirit thereof as defined by the appended claims. The specification and drawing are accordingly to be regarded in an illustrative rather than a limiting sense.

I claim:

1. A frequency doubler comprising an'electron discharge tube having at least a cathode, an anode and a pair of control grids, a source of alternating potential, means for exciting said control grids by potentials in quadrature phase relation derived from said source, an output circuit for said tube, and means for utilizing energy developed in said output circuit having a frequency twice the frequency of said source.

2. A frequency doubler comprising an electron discharge tube having a cathode, an anode and a pair of control elements adapted to produce anode current variations proportional to the product of the potentials exciting said control elements, a source of alternating potential, means including phase shifting meansfor impressing potentials in quadrature phase'relation from said source upon said control elements, and means for utilizing energy developed in said output circuit having a frequency twice the frequency of said source.

3. A frequency doubler comprising an electron discharge tube having at least a cathode, an anode, a pair of control grids and an acceleration grid between said control grids, means for maintaining said anode and acceleration grid at positive direct current operating potentials with respect to said cathode, a source of alternating potential, means including phase shifting means for exciting said control'grids by potentials in quadrature phase relation derived from said source, an output circuit connected to said anode and a parallel tuned circuit resonant to twice the frequency of said source inserted in said output circuit.

4. A frequency doubler comprising an electron discharge tube having at least a cathode, an anode, a pair of control grids and an acceleration grid between said control grids, means for maintaining said anode and said acceleration grid at positive direct current operating potentials with respect to said cathode, a source of alter- .nating potential, means for impressing potential from said source upon both said control grids, phase shifting means between said source and at least one of said control grids whereby the alternating potentials on said control grids are in phase quadrature, an output circuit connected to said anode, and a parallel tuned circuit resonant at twice the frequency of said source inserted in said output circuit.

5. A frequency doubler comprising an electron discharge tube having at least a cathode, an anode, a pair of control grids and an acceleration grid between said control grids, means for maintaining said anode and said acceleration grid at positive direct current operating potentials with respect to said cathode, a source of alternating potential, a double tuned transformer connected to said source, said transformer having primary and secondary tuned circuits resonant to the frequency of said source, means for impressing potential developed by the primary of said transformer upon one of said control grids and for impressing potential developed by the secondary of said transformer upon the other of said control grids, an output circuit connected to said anode, and a parallel tuned circuit resonant to twice the frequency of said source inserted in said output circuit.

6. A frequency doubler comprising an electron discharge tubehaving at least a cathode, a pair of control grids, an acceleration grid between said control grids and an anode, means for maintaining said anode and said acceleration grid at positive direct current operating potentials with respect to said cathode, a source of alternating potential, means for exciting one of said control grids from said source,impedance means offering high resistive impedance at least to currents having a frequency equal to the frequency of 'said source, said impedance means being connected to said other control grid and cathode, an output circuit connected to said anode, and a parallel tuned circuit resonant to twice the frequency of said source inserted in said output circuit.

'7. A frequency doubler comprising an electron discharge tube having at least a cathode, a pair of control grids, an acceleration grid between said control grids and an anode, a source of alternating potential, means including circuit connections for exciting both said control grids from said source, a network comprising resistive and reactive impedance connected between said source and at least one of said control grids whereby the exciting potentials on said control grids are in phase quadrature, an output circuit connected to said anode, and a parallel tuned circuit resonant to twice the frequency of said source inserted in said output circuit.

8. A frequency doubler comprising a source of alternating potential, intermodulating means K having a pair of input circuits and an output circuit and adapted to produce output current variations being proportional to the product of potentials impressed upon said input circuits,

the

means including phase shifting means for impressing potentials from said source in phase quadrature upon said input circuits, and means for utilizing energy developed in said output circuit having a frequency twice the frequency of said source.

9. A frequency doubler comprising a source of alternating potential, intermodulating means having a pair of input circuits and an output cirof said source inserted in said output circuit.

10. A frequency doubler comprising an electron discharge tube provided with means for producing an electron space current, a source of alternating potential, means including circuit connections from said source to said tube for varying said space current in accordance with said alternating potential, a control electrode in said tube, means for producing a concentrated electron space charge adjacent to said control electrode, impedance means being substantially non-reactive to at least the frequency of said source connected to said control electrode to develop a quadrature alternating potential thereon by the displacement current induced in said impedance means by said space charge, an output circuit for said tube and means for utilizing energy developed in said output circuit having a frequency twice the frequency of said alternating potential.

11. A frequency doubler comprising an electron discharge tube provided with means for producing an electron space current, a source of alternating potential, means. including circuit connections from said source to said tube for varying said space current in accordance with said alternating potential, a control electrode in said tube, means for producing a concentrated electron space charge adjacent to said control electrode, a parallel tuned circuit being resonant to the frequency of said alternating potential and connected to said control electrode to develop a quadrature alternating potential thereon by the displacement current induced in said circuit by said space charge, an output circuit for said tube, and a parallel tuned lead circuit resonant to twice the circuit.

12. A frequency doubler comprising an electron discharge tube provided with a cathode, an accelerating grid and a control grid arranged in the order named, a source of alternating potential, means including circuit connections from said source in said tube for varying the electron current emitted from said cathode in accordance with said alternating potential, means for maintaining said accelerating grid at a steady positive potential with respect to said cathode, further means for maintaining said control electrode at a steady potential substantially negative with respect to said accelerating grid to produce a concentrated electron space charge adjacent to said control electrode, impedance means being substantially non-reactive to at least the frequency of said alternating potential and connected between said control electrode and cathode to develop a quadrature alternating potential upon said control electrode by the displacement current induced in said impedance means by said space charge, an output circuit for said tube, and means for utilizing energy developed in said output circuit having a frequency twice the frequency of said source.

13. A. frequency doubler comprising an electron discharge tube provided with a cathode, an accelerating grid and a control electrode arranged in the order named,a source of alternating potential, means including circuit connections from said source to said tube for varying the electron current emitted from said cathode in accordance with said alternating potential, means for maintaining said accelerating grid at a steady'positive potential with respect tosaid cathode, further means for maintaining said control electrode at a steady potential substantially negative with respect to said accelerating grid to produce a concentrated electron space charge in the vicinity of said control electrode, a parallel tuned circuit being resonant to the frequency of said alternate ing potential and connected between said control electrode and cathode to develop a quadrature alternating potential upon said control electrode by the displacement current induced in said circuit by said space charge, an output circuit for said tube, and a parallel tuned load circuit resonant to twice the frequency of said alternating potential inserted in said output circuit.

HENRY M. BACH. 

